The human intervertebral junction is characterized principally by an intervertebral disc interposed between adjacent vertebral surfaces. The size and configuration of discs vary between the six discs of the cervical region, the twelve discs of the thoracic region, six of the lumbar region and one disc between the sacrum and coccyx.
Intervertebral discs are neither homogeneous nor static. Changes to a disc can affect the vertebral column activity significantly. The intervertebral disc is a complex structure where its dynamic properties result from the interaction of a central, gelatinous nucleus pulposus encircled by a tough, fibrous, semielastic annulus fibrosus. Further, thin cartilage endplates and vertebral body ring apophyseal attachments of the annulus fibrosus join the disc to the vertebrae craniad and caudad to the disc. Although the nucleus pulposus is gelatinous and somewhat fluid while the annulus fibrosus comprises circularly arranged fibers, the border between these components is not distinct in a healthy adult disc. Any distinction is less apparent in a damaged disc where tissues are intermingled in a gradual transition layer.
The annulus fibrosus is composed of concentric layers of fibrocartilage, in which collagen fibers are arranged in parallel strands running obliquely between vertebral bodies. The inclination is reversed in alternate layers thereby crossing over each other obliquely. In children and adolescents, the nucleus pulposus is an amorphous colloidal mass of gelatinous material containing glycosaminoglycans, collagen fibrils, mineral salt, water and cellular elements. The nucleus pulposus has an important function in nutrition of the disc and contributes to the mechanical ability of the disc to act as a shock absorber and allow flexibility. The nucleus pulposus is normally under pressure and is contained within an ovoid cavity formed laterally by the annulus fibrosus and bounded by thin plates of hyaline cartilage endplates covering the adjacent vertebrae.
The intervertebral discs form about one-quarter the length of the vertebral column in a healthy adult human. Discs are thickest in the cervical and lumbar regions, where the movements of the vertebral column are greatest. The vertebral column, including the intervertebral discs, undergo various morphological and biochemical changes over time, such as dehydration of the discs and concaving vertebral bodies. As a result, the size and configuration of the disc components vary considerably from person to person.
Lower back injuries and chronic back pain are a major health problem resulting not only in a debilitating condition for the patient, but also in the consumption of a large proportion of funds allocated for health care, social assistance and disability programs. Disc abnormalities and pain may result from trauma, repetitive use in the workplace, metabolic disorders, inherited proclivity or aging. The existence of adjacent nerve structures and innervation of the disc are very important issues in respect to patient treatment for back pain.
Common disorders of the intervertebral disc include localized tears or fissures in the annulus fibrosus; disc herniations with contained or escaped extrusions of the nucleus pulposus; and chronic circumferential bulging of discs. For most patients, however, a well-defined abnormality cannot be found to solely explain the cause of the low back pain, making treatment and pain management very difficult. Since isolating a specific anatomic disorder as the sole cause of pain is rare, most patients are merely treated symptomatically to reduce pain, rather than receiving treatment to eliminate the cause of the condition.
One course of pain may be attributed to the structure of the annulus fibrosus. The annulus fibrosus is thinner nearer to the posterior than to the anterior margin of the disc, and many disc ruptures occur in the posterior region thereby exerting pressure on the adjacent nerve fibers causing pain. The pain experienced by the disc exerting pressure on the adjacent nerves is characterized by referred pain, or pain felt predominantly elsewhere in the body where the affected nerve travels. A common example of this is sciatica where an intervertebral disc exerts pressure on the sciatic nerve.
Another cause of pain resulting from disc pathology is chemically-induced pain. The nucleus pulposus contains chemicals that may induce pain if contact is made with certain nerve structures. If an intervertebral disc is herniated severely enough that a portion of the nucleus pulposus is extruded from the disc, and the portion comes in contact with an adjacent nerve, chemically-induced pain can be felt. This is also a cause of sciatica.
Increasingly, evidence suggests that the source of back pain in many patients is a result of nerves within the degenerated disc itself or nerves that have grown into the disc in concordance with disc injury. For example, as documented by Jonathan C. Houpt, BA, Edison S. Conner, MD, and Eric W. McFarland in “Experimental Study of Temperature Distributions and Thermal Transport During Radio frequency Current Therapy of the Intervertebral Disc”, Spine. 1996; 21(15), 1808-1813, afferent innervation of the outer half of the annulus fibrosus has been established whereas the nucleus pulposus contains no nerves or blood vessels. Pain response has been widely reported in response to specific stimulation of the outer layers of the annulus fibrosus. In another study documented by A. J. Freemont, “Nerve ingrowth into diseased intervertebral disc in chronic back pain”, The Lancet. 1997; 350, 178-181, nociceptive nerves were found ingrown deeper into the disc, as far as the nucleus pulposus, in association with disc degeneration. The pain experienced from nerves in a damaged intervertebral disc is more localized to the spine. The stimulation can be both mechanical and chemical. Some patients may feel a combination of back pain and referred pain indicating that pain is being transmitted both from nerves in the disc and from impinged nerves adjacent to the disc. It appears that the disc is devoid of temperature-sensing neurological structures, possibly due to the fact that the disc is at core body temperature, and only mechanical and chemical stimulus-sensing nociceptors exist in the disc.
Where patients are diagnosed with clear chronic discogenic pain (i.e. pain originating from a disc), complete surgical removal of the intervertebral disc (called discectomy) and fusion of the adjacent vertebrae is often carried out with success rates over 80% in measurable pain reduction after surgery. Such major surgical procedures are highly invasive, expensive and involve significant risk. Furthermore motion is impeded once the vertebrae are fused and there may be adverse mechanical effects on the adjacent remaining discs.
To alleviate some of the disadvantages of open-surgery discectomy, percutaneous methods of removing the disc or part of the disc have been practiced. Methods that remove part of the nucleus pulposus are designed to decrease the volume in order to reduce internal disc pressure thus reducing external pressure exerted on adjacent nerves. Examples of such methods that include mechanical means can be found in, for example, U.S. Pat. No. 4,369,788 to Goald that describes the use of a mechanical device for use in microlumbar discectomy, and in U.S. Pat. No. 5,201,729 to Hertzmann et al. that describes a percutaneous method of discectomy using a laser. Other methods of removing the disc or part of the disc include chemically dissolving the nucleus pulposus using the enzyme Chymopapain. U.S. Pat. No. 6,264,650 to Hovda et al. describes a method of vaporizing a portion of the nucleus pulposus using radio frequency electrical current. These prior art methods have shown variable success and there are several advantages of percutaneous procedures over open surgical discectomy and vertebral fusion including less trauma to the patient, preserved spinal movement, less disruptive effect on adjacent discs, less risk of infection and less risk of accidental injury. However, these methods involve removing a portion of the nucleus pulposus, which is essential to the maintenance of the disc. Further, the damaged annulus fibrosus is not treated.
Due to the pain reduction success of surgical discectomy, less drastic means of denervating rather than surgically removing the disc are of significant interest. To denervate is to intervene with the transmission of a sensory signal in a nerve. A denervated disc does not cause discogenic pain and the disc is left intact to preserve its mechanical function. Denervating the disc especially by using percutaneous probes is much less invasive, less costly and less risky. The procedure is also simpler to administer and does not require the fusing of adjacent vertebrae thereby better preserving the patient's freedom of movement.
A minimally invasive technique of delivering high-frequency electrical current has been shown to relieve localized pain in many patients. Generally, the high-frequency current used for such procedures is in the radio frequency (RF) range, i.e. between 100 kHz and 1 GHz and more specifically between 300-600 kHz. The RF electrical current is typically delivered from a generator via connected electrodes that are placed in a patient's body, in a region of tissue that contains a neural structure suspected of transmitting pain signals to the brain. The electrodes generally include an insulated shaft with an exposed conductive tip to deliver the radio frequency electrical current. Tissue resistance to the current causes heating of tissue adjacent resulting in the coagulation of cells (at a temperature of approximately 45° C. for small unmyelinated nerve structures) and the formation of a lesion that effectively denervates the neural structure in question. Denervation refers to a procedure whereby the ability of a neural structure to transmit signals is affected in some way and usually results in the complete inability of a neural structure to transmit signals, thus removing the pain sensations. This procedure may be done in a monopolar mode where a second dispersive electrode with a large surface area is placed on the surface of a patient's body to complete the circuit, or in a bipolar mode where a second radio frequency electrode is placed at the treatment site. In a bipolar procedure, the current is preferentially concentrated between the two electrodes.
In order to extend the size of a lesion, radio frequency treatment may be applied in conjunction with a cooling mechanism, whereby a cooling means is used to reduce the temperature of the tissue in the vicinity of an energy delivery device, allowing a higher voltage to be applied without causing an unwanted increase in local tissue temperature. The application of a higher voltage allows regions of tissue further away from the energy delivery device to reach a temperature at which a lesion can form, thus increasing the size/volume of the lesion
The treatment of pain using high-frequency electrical current has been applied successfully to various regions of patients' bodies suspected of contributing to chronic pain sensations. For example, with respect to back pain, which affects millions of individuals every year, high-frequency electrical treatment has been applied to several tissues, including intervertebral discs, facet joints, sacroiliac joints as well as the vertebrae themselves (in a process known as intraosseous denervation). In addition to creating lesions in neural structures, application of RF energy has also been used to treat tumors throughout the body.
In an effort to reduce back pain through early intervention techniques, some investigators have focused upon nerves contained within the vertebral bodies which are adjacent to the intervertebral discs. For example, in PCT Patent Publication No. WO 01/0157655, Heggeness discloses ablating nerves contained within the vertebral body (intraosseous nerves) by first boring into the vertebral body with a nerve ablation device, placing the tip of the device in close proximity to the nerve, and then ablating the nerve using the tip. However, previous techniques fail to describe how to effectively carry out nerve ablation when the precise location of the intraosseous nerve is unknown, or when the electrode tip cannot be manoeuvred relatively close to the intraosseous nerve.
With respect to the intervertebral disc itself, U.S. Pat. No. 5,433,739 to Sluijter et al. describes a method of relieving back pain through percutaneous insertion of a needle or electrode into the center of the intervertebral disc within the nucleus pulposus under fluoroscopy or other imaging control. The U.S. Pat. No. 5,433,739 describes the heating of the outer layers of the annulus fibrosus to a temperature that is lethal to the nerve structures thereby denervating the disc to relieve discogenic pain. The temperature of the tissue is increased by applying high frequency electric current through the tissue.
It is well known to those skilled in the art that percutaneous access to an intervertebral lumbar disc involves either a posterolateral approach or an anterior approach. The anterior approach is more invasive than the posterolateral approach because of the organs in the abdominal and pelvic cavities. The most common percutaneous approach to the lumbar disc, to those skilled in the art, is to insert a needle or tube posterolateral to the disc, just lateral of the zygapophyseal joint, inferior to the spinal nerve and into the posterolateral region of the annulus fibrosus.
In accordance with U.S. Pat. Nos. 5,980,504; 6,007,570; 6,073,051; 6,095,149; 6,099,514; 6,122,549; 6,126,682; 6,258,086 B1; 6,261,311 B1; 6,283,960 B1; and 6,290,715 B1 (“the Sharkey et al. patents”) to Sharkey et al. to permit percutaneous access to the posterior half of the nucleus or to the posterior inner wall of the disc, a flexible heating element may be inserted into the nucleus pulposus through a hollow tube that has been inserted through the annulus fibrosus. The flexible heating element has sufficient rigidity to be advanced longitudinally under force through the nucleus pulposus while having sufficient flexibility to be compliant to the inner wall of the annulus fibrosus. The heating element is guided by sliding contact with the inner wall and ideally should not puncture or damage the annulus fibrosus during positioning. Another embodiment disclosed in U.S. Pat. No. 6,258,086 B1 is a flexible probe that contains an activation element on the distal portion that changes the shape of the probe once it is in the nucleus pulposus. According to the Sharkey et al. patents, the flexible heating elements operate to denervate the outer layers of the annulus fibrosus as well as modulate the collagen in the annulus fibrosus by applying heat. Raising the temperature above about 60° C. will break structural bonds of collagen fibers causing them to contract and tighten. This collagen-tightening effect is lost once the temperature of the collagen is raised above about 75° C. where the fibers loosen, resulting in zero net volume change.
There is interest among researchers that the application of high frequency current without a rise in temperature alters nerve function to relieve pain. Use of high frequency current without heating to relieve pain by modifying neural tissue is described in U.S. Pat. Nos. 5,983,141; 6,161,048; 6,246,912; and 6,259,952 (“the Sluijter et al. patents”) to Sluijter et al. These patents describe the use of a modified signal wave that includes rest periods to allow heat to dissipate. The modified high frequency signal is applied to the patient using a single active electrode and a ground electrode attached to the skin of the patient. These disclosures (the Sluijter et al. patents) do not discuss using high frequency current to increase collagen production nor do they discuss this application in the intervertebral disc. The disclosures that are specifically designed for treatment of intervertebral discs (the Sharkey et al. patents; U.S. Pat. No. 5,433,739 of Sluijter et al.; and Finch PCT publication number WO 01/45579) do not discuss the application of high frequency current without a rise in temperature to alter nerve function to relieve pain or to cause collagen production to increase. The advantages of non-thermal application of high frequency electrical current to treat intervertebral discs include reduced risk of thermal damage, increased production of collagen to strengthen the annulus fibrosus, and reduced discogenic pain while stimulating the healing processes.
The above referenced publications describe the use of monopolar devices for treatment procedures and are therefore restricted by the limitations of using a monopolar probe. For example, since energy is primarily concentrated around the lone electrode in a monopolar device, precise knowledge of the location of the tissue to be treated is required. In contrast, in a bipolar procedure, the energy is concentrated between two electrodes allowing a tissue to be affected by the treatment procedure provided it is located substantially between the electrodes. The use of two electrodes in a bipolar configuration also allows for the creation of a more uniform lesion than with a single electrode where the energy is concentrated at the surface of the electrode.
There is a continued need for improvement in systems used for RF treatment of bodily tissue, and it would be beneficial to have a device and a system that is improved and/or overcomes some or all of the limitations of the prior art.